Treatment of Silica Based Soot or an Article made of Silica Based Soot

ABSTRACT

One embodiment of the disclosure relates to a method of cleaning silica-based soot or an article made of silica-based soot, the method comprising the step of treating silica-based soot or the article made of silica-based soot with at least one of the following compounds: (i) a mixture of CO and Cl 2  in a carrier gas such that the total concentration of CO and Cl 2  in the mixture is greater than 10% (by volume, in carrier gas) and the ratio of CO:Cl 2  is between 0.25 and 5; (ii) CCl 4  in a carrier gas, such that concentration CCl 4  is greater than 1% (by volume, in carrier gas). Preferably, the treatment by CCl 4  is performed at temperatures between 600° C., and 850° C. Preferably, the treatment with the CO and Cl mixture is performed at temperatures between 900° C. and 1200° C. The carrier gas may be, for example, He, Ar, N 2 , or the combination thereof.

CROSS-REFERENCE

This application claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 61/373,005 filed on Aug. 12, 2010 the content of which is relied upon and incorporated herein by reference in its entirety.

BACKGROUND

The disclosure relates generally to a method of decreasing the concentration of impurities in silica soot, and more particularly to a method of decreasing the concentration of ZrO₂ and Cr₂O₃ particles that are present (i) in soot which may be used in manufacturing processes that utilize compressed soot or fumed silica in optical preform manufacturing, or in manufacturing of other articles, (ii) in glass or silica soot articles such as optical preforms.

Optical soot preforms are used in making optical fibers. These soot preforms are sintered into glass and then drawn into optical fibers. One method of making soot preforms includes pressing of the fumed silica or reclaimed soot around a core cane or around an OVD soot covered core cane. The pressed soot preform is further heat-treated, dehydrated and cleaned of some impurities by treatment with chlorine (Cl₂, typically for about 4 hrs) at about 3% concentration by volume. The “cleaned” preform is sintered into a glass blank, and then drawn into optical fiber.

During the making of the optical preforms, ZrO₂ and Cr₂O₃ particles are often present in the soot and thus are incorporated into the preform. These refractory particles have melting temperatures that are much higher than the draw temperatures and can be the sources for fiber breaks if left incorporated in the fiber. The sizes of these refractory particles have to be below a certain size (e.g., below 0.2 or 0.1 μm) for them to not cause fiber breaks under tensile loading. If the initial size of the Cr₂O₃ particles is relatively small (less than 0.25 μm), this can be done by treating preforms with dehydration agents such as chlorine. However, Cl₂ cleaning is ineffective at removing ZrO₂ particles, or for larger sized Cr₂O₃ particles. That is, ZrO₂ and larger Cr₂O₃ particles (i.e., 0.5 μm or larger in cross-section) that are present in the soot or in optical soot preforms cannot be eliminated or sufficiently decreased in size by using standard Cl₂ treatment.

However, particle impurities for certain processes (soot pressing, for example) can be larger in size and can be present in significant concentrations, making them difficult to remove using a standard treatment with Cl₂.

No admission is made that any reference cited herein constitutes prior art. Applicant expressly reserves the right to challenge the accuracy and pertinence of any cited documents.

SUMMARY

One embodiment of the disclosure relates to a method of cleaning silica-based soot or an article made of silica-based soot, the method comprising the step of:

treating silica-based soot or the article made of silica-based soot with at least one of the following compounds:

-   -   (i) a mixture of CO and Cl₂ in a carrier gas such that the total         concentration of CO and Cl₂ in the mixture is greater than 10%,         by volume and the ratio of CO:Cl₂ is between 0.25 and 5;     -   (ii) CCl₄ in a carrier gas, such that concentration CCl₄ is         greater than 1% by volume.

Preferably, the treatment by CCl₄ is performed at temperatures between 600° C., and 850° C. Preferably, the treatment with the CO and Cl mixture is performed at temperatures between 900° C. and 1200° C. Preferably, the treatment by CCl₄ is performed for at least 20 min, more preferably for at least 50 min Preferably, the treatment with the CO and Cl₂ mixture is performed for at least 30 min, more preferably for at least 100 min.

An additional embodiment of the disclosure relates to a method of cleaning silica-based soot or an article made of silica-based soot, the method comprising the following steps:

-   -   treating said silica-based soot or said article made of         silica-based soot with     -   (i) CCl₄ in a carrier gas, such that concentration CCl₄ is         greater than 1%, by volume; and     -   (ii) a mixture of CO and Cl₂ in a carrier gas such that the         total concentration of CO and Cl₂ in said mixture is greater         than 10%, by volume and the ratio of CO:Cl₂ is between 0.25 and         5;         -   wherein said treatment with CCl₄ is performed either before,             or after the treatment with the mixture of CO and Cl₂.

Preferably, the treatment with CCl₄ is performed at temperatures between 600° C., and 850° C., and the treatment with the mixture of CO and Cl is performed at temperatures between 900° C. and 1200° C.

The method(s) disclosed herein can advantageously remove refractory particles from silica-based soot and silica-based soot articles (e.g., optical soot preforms). Another advantage of the method(s) disclosed herein is that treatment of silica-based soot preforms lowers the concentration and/or or eliminates the presence of undesirable refractory particles, which that can result in elimination of these particles as sources of fiber breaks. Another advantage of the method(s) disclosed herein is that after the treatment of silica-based soot preforms, the larger particles are decreased in size such that they are no longer sources of fiber breaks. For example 1 μm, 2 μm, 5 μm, or even 10 μm Cr₂O₃ and/or ZrO₂ particles can be decreased in size to 0.2 um, or 0.1 um, or smaller.

Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims hereof, as well as the appended drawings.

It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understand the nature and character of the claims.

The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the description serve to explain principles and operation of the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the decrease in Cr₂O₃ particle size (initial size=0.25 μm) at 1100° C. at different concentrations of Cl₂;

FIG. 2 illustrates the decrease in Cr₂O₃ particle size (initial size=0.25 μm) at 1100° C., at different concentrations of Cl₂ and CO, according to one embodiment;

FIG. 3 illustrates the decrease in Cr₂O₃ particle size (initial size=0.25 μm) with time, when soot is treated, according to at least one embodiment, at three different temperatures with a mixture of CO and Cl₂;

FIG. 4A illustrates the decrease in Cr₂O₃ particle size versus time, for different initial particle sizes when soot is treated, according to one embodiment, at 1100° C. with a mixture of CO and Cl₂;

FIG. 4B illustrates the decrease in Cr₂O₃ particle size versus time, for different initial particle sizes when soot is treated, according to one embodiment, at 1175° C. with a mixture of CO and Cl₂;

FIG. 4C illustrates the decrease in Cr₂O₃ particle size versus time, for different initial particle sizes when soot is treated according to one embodiment, at 1175° C. with a mixture of CO and Cl₂, but at higher concentrations compared to FIG. 4B.

FIG. 5 illustrates the decrease in ZrO₂ particle size (initial size=1 μm) after treatment according to at least one embodiment, with different concentrations of CCl₄, at 527° C.;

FIG. 6 illustrates the decrease in ZrO₂ particle size (initial size=1 μm) after treatment according to at least one embodiment, with different concentrations of CCl₄, at 627° C.;

FIG. 7 illustrates the decrease in ZrO₂ particle size (initial size=1 μm) after treatment according to at least one embodiment, with CCl₄ at three different temperatures; and

FIG. 8 illustrates the decrease in ZrO₂ particle size versus time, for different initial particle sizes, when soot is treated according to one embodiment at 727° C. with CCl₄.

DETAILED DESCRIPTION

Various embodiments will be further clarified by the following examples.

One embodiment of the disclosure relates to a method of cleaning of silica-based soot or an article made of silica-based soot. According to this embodiment the method comprises the step of:

treating silica-based soot or the article made of silica-based soot with at least one of the following compounds:

-   -   (i) a mixture of CO and Cl in a carrier gas such that the total         concentration of CO and Cl₂ in said mixture is greater than 10%,         by volume and the ratio of CO:Cl₂ is between 0.25 and 5;     -   (ii) CCl₄ in a carrier gas, such that concentration CCl₄ is         greater than 1% by volume, in carrier gas. For example, CCl₄         concentration in carrier gas may be 1.5%, 2 vol %, 3 vol %, 5         vol %, 10 vol %, 15 vol %, 20 vol %, 25 vol %, 30 vol %, 35 vol         %, or 40 vol %.) Preferably, the treatment by CCl₄ is performed         at temperatures between 600° C., and 850° C. Preferably, the         treatment with the CO and Cl mixture is performed at         temperatures between 900° C. and 1200° C., more preferably,         between 1000° C. and 1200° C., and even more preferably between         1100° C. and 1200° C. (e.g., 1050° C., 1075° C., 1100° C., 1125°         C., 1150° C., 1175° C., or 1190° C.). The carrier gas may be,         for example, He, Ar, N₂, or the combination thereof.

Preferably, the treatment by CCl₄ is performed for at least 2 min, more preferably for at lest 5 min, more preferably at lest 10 or 20 min, for example for at least 50 min. Preferably, the treatment with the CO and Cl₂ mixture is performed for at least 5 min, preferably at least 20 min or 30 min, more preferably for at least 100 min. Preferably, the ratio of CO to Cl₂ is between 0.5 and 2, more preferably between 0.75 and 1.5.

According to one embodiment the method of cleaning silica-based soot or an article made of silica-based soot comprises the following steps:

treating said silica-based soot or said article made of silica-based soot with

-   -   (i) CCl₄ in a carrier gas, such that concentration CCl₄ is         greater than 1%, by volume; and     -   (ii) a mixture of CO and Cl₂ in a carrier gas such that the         total concentration of CO and Cl₂ in said mixture is greater         than 10%, by volume and the ratio of CO:Cl₂ is between 0.25 and         5;         -   wherein the treatment with CCl₄ is performed either before,             or after the treatment with the mixture of CO and Cl₂.             Preferably, the treatment by CCl₄ is performed at             temperatures between 600° C., and 850° C., and the treatment             with the mixture of CO and Cl₂ is performed at temperatures             between 900° C. and 1200° C. (e.g., 950° C., 975° C., 1000°             C., 1025° C., 1050° C., 1075° C., 1100° C., 1125° C., 1150°             C., 1175° C., 1190° C.). Preferably the treatment with CCl₄             is performed for at least 2 min, and the treatment with a             mixture of CO and Cl₂ is performed for at least 5 min.

FIG. 1 illustrates the decrease in Cr₂O₃ particle size at three different concentrations of chlorine (Cl₂ was utilized alone, without CO), at the treatment temperature of 1100° C. The initial size of the Cr₂O₃ particles was 0.25 μm. In these three examples the Cl₂ concentration in the carrier gas was 5%, 10%, and 25% by volume respectively, which is higher than the typically used concentrations of Cl₂ when treating optical preforms.

FIG. 1 illustrates that even when the initial Cr₂O₃ particle sizes are relatively small (about 0.25 μm) the amount of time needed to eliminate the Cr₂O₃ particles is relatively long even when high concentration of chlorine (about 10%) is used at high temperatures (about 1100° C.). For example, FIG. 1 shows that at a Cl₂ concentration of 10%, the required treatment time to eliminate 0.25 μm Cr₂O₃ particles is approximately 5 hrs (300 min). Even when the Cl₂ concentration is 25%, the required treatment time to eliminate 0.25 μm Cr₂O₃ particles is approximately 2 hrs.

As shown in FIG. 2, the soot treatment time is dramatically decreased when a mixture of carbon monoxide and chlorine is utilized. For example, FIG. 2 illustrates that the addition of 10% CO to 10% of Cl₂ in a carrier gas results in a ten-fold decrease in treatment time (from about 300 min, as shown in FIG. 1, to about 30 min). The initial particle sizes of Cr₂O₃ particles of FIG. 2 is identical to those of FIG. 1 (about 0.25 μm), and the treatment temperature is also the same (about 1100° C.). FIG. 2 illustrates that the shortest treatment time is achieved when the ratio of chlorine to carbon monoxide is 1:1. It is noted that when the concentration of Cl₂ is increased (e.g., to about 15% (0.15 atm), 20% (0.2 atm), 25% (0.25 atm)), with the corresponding increase n CO concentration, the treatment time for soot containing Cr₂O₃ particles may be decreased even further.

The particle size decrease rates at different temperatures, when silica soot is treated with a CO and Cl₂ mixture at 10% concentrations of each CO and Cl₂ is shown in FIG. 3. For example, when the treatment (particle processing) temperatures is 900° C., the treatment time for the elimination of metal oxide particles such as Cr₂O₃, when the initial particle sizes are about 0.25 μm, is about 140 min-150 min. When the treatment temperature is 1100° C., the treatment time for elimination of metal oxide particles, such as Cr₂O₃, is about 30-35 min (when the initial particle sizes are 0.25 μm). Thus, in order to decrease the silica soot process treatment time when using the CO/Cl₂ mixture, it is advantageous to have the reaction(s) at temperatures greater than 900° C., more preferably greater than 1000° C. and even more preferably greater than 1100° C. (e.g., 1125° C., 1150° C., 1175° C., 1190° C.). However, at temperatures above 1200° C., there can be significant etching of the silica soot by the CO/Cl₂ mixture and some sintering of the soot preform. Consequently, the maximum temperature is preferably not higher than 1200° C.

The rate of particle size decrease for different initial particle size is illustrated in FIG. 4A. We discovered that when Cr₂O₃ particles are large enough such that their presence in the soot preform can later result in fiber breaks, the treatment time needs to be chosen appropriately, based on the maximum particle size. For example, if one knows that silica soot contains Cr₂O₃ particles that are up to 2 μm in diameter, (if the treatment temperature is 1100° C.) the processing time with the CO/Cl₂ mixture (10%/10% vol % in carrier gas) should be about 4 to 4.5 hrs. If the Cr₂O₃ particles have a maximum diameter of 5 μm, the processing time with the CO/Cl₂ mixture (same concentration) should be about 10-12 hrs (if the treatment temperature is 1100° C.). However, these processing times can be decreased if the temperature during CO/Cl₂ is increased to 1150° C.-1175° C. It is noted for that when the concentration of Cl₂ is increased (e.g., to about 15%, 20%, or 25%) with the corresponding increase in CO concentration, such that the ratio of CO:Cl₂ is between 0.5 and 2 (preferably, between 0.75, and more preferably 1) the treatment time for soot containing larger Cr₂O₃ particles (e.g., 1-10 μm diameter, or larger) may be decreased even further. FIG. 4B illustrates the rate of particle size decrease for different initial particle size when the treatment temperature is 1175° C. (same concentrations (10 vol % of Cl₂ and 10 vol % of CO, in carrier gas). FIG. 4C illustrates the rate of particle size decrease for different initial particle size when the treatment temperature is 1175° C., but the concentrations Cl₂ and CO (in carrier gas) were each increased to 30 vol % (0.3 atm).

It is noted that the four plots in FIGS. 4A-4C are parallel to one another. Thus, to determine the appropriate treatment time for the soot containing maximum particle sizes other than those depicted in FIGS. 4A-4C, one needs to simply draw line parallel to those shown in FIG. 4A-4C, but starting at the appropriate particle size level. Similarly, if other metal oxides are present in the soot, or if using uses different concentrations of Cl₂ and CO, one can create a graph similar to that of FIG. 4A-4C, to determine the appropriate treatment time for the metal oxide particles of the specific compositions that are present in the soot and that are present in sizes that are large enough that requires either their elimination, or the decrease in size of these particles below a certain level.

We discovered that for Cr₂O₃ particles of size d_(p) (cross sectional length, or diameter, in μm) treated at temperature T (in ° K) with mixture of chlorine and carbon monoxide gases having chlorine and carbon monoxide concentrations of y_(Cl2) and y_(CO) (in atm), the treatment time should be greater than the diffusion time and reaction time, as shown below, i.e.:

t _(treatment,Cr2O3) (in mm)>t _(diffusion) +t _(reaction,Cr2O3)  [1]

where the diffusion reaction time is a function of soot layer thickness L (in cm) and diffusion rate of the CO/Cl₂ mixture D_(eff) (in cm2/sec) through the porous soot preform, or loose silica soot and is given as:

$\begin{matrix} {{t_{diffusion}\left( {{in}\mspace{14mu} \min} \right)} = \frac{L^{2}}{60D_{eff}}} & \lbrack 2\rbrack \end{matrix}$

and the reaction time is given as:

$\begin{matrix} {{t_{{reaction},{{Cr}\; 2O\; 3}}\left( {{in}\mspace{14mu} \min} \right)} = \frac{4.3 \times 10^{- 4}\left( {d_{p}\left( {{in}\mspace{14mu} {\mu m}} \right)} \right){{Exp}\left\lbrack {12000/{T\left( {{in}\mspace{14mu} K} \right)}} \right\rbrack}}{y_{{Cl}\; 2}{x_{{Cl}\; 2}\left( {1 - {1.35x_{{Cl}\; 2}} + {0.372x_{{Cl}\; 2}^{2}}} \right)}}} & \lbrack 3\rbrack \end{matrix}$

Methods to find the effective diffusion rate (D_(eff)) of a gas in porous soot preform are known in the art. In Eq. [3], parameter X_(Cl2) is given as x_(Cl2):(y_(Cl2))/(y_(Cl2)+y_(CO)), where y_(cl2) and y_(co) are the partial pressure of chlorine and carbon monoxide respectively. According to some embodiments the treatment time with the CO/Cl₂ mixture is greater than 5 min, for example 10 min to 100 hrs. Preferably the treatment time with the CO/Cl₂ mixture is 10 min to 15 or 20 hrs, for example 20 min, 30 min, 50 min, 1 hr, 1.5 hrs, 2 hrs, 3 hrs, 4 hrs, 6 hrs, 8 hrs, 10 hrs, 12 hrs., or some period of time there between.

If ZrO₂ particles are present in the soot, chemical treatment of soot with chlorine and CO mixture is not very effective. That is, although some ZrO₂ particles will be eliminated or minimized in size to the appropriate level during the treatment with the CO/Cl₂ mixture as described above, not all of the ZrO₂ particles will be decreased in size to the appropriate levels (0.1 μm, or smaller). However we discovered that the treatment of ZrO₂ particles with CCl₄ is very effective. We have found that CCl₄ treatment of silica soot, for example of optical preforms containing porous silica soot is most effective in temperature range between 600° C.-850° C. Below 600° C., the reaction of CCl₄ is slow and above 850° C., CCl₄ can decompose and react with silica muffle and silica soot particles, as well as form elemental carbon.

The rate of ZrO₂ particle size decrease at different CCl₄ concentrations, temperatures and for the specified initial ZrO₂ particle sizes is shown in FIGS. 5-8. More specifically, FIGS. 5 and 6 illustrate that the decrease in ZrO₂ particle size occurs faster with higher concentrations of CCl₄ and at higher temperatures. For example, as shown in FIG. 5, at a treatment temperature of 527° C., when the concentration CCl₄ was increased from 10% to 30% (0.1 atm to 0.3 atm) the complete elimination of ZrO₂ particles up to 1 μm in cross-section occurred in 62 min instead of 185 min. As shown in FIG. 6, at a treatment temperature of 627° C., the treatment proceeded much faster than at 527° C., and when the concentration of CCl₄ was increased from 10% to 30% (0.1 atm to 0.3 atm) the complete elimination of ZrO₂ particles up to 1 μm in cross-section occurs in less than 15 min, instead of 35 min. FIG. 7 illustrates the decrease in ZrO₂ particle size (initial size=1 μm) at different temperatures, for CCl₄ concentrations of 10 vol %, in carrier gas (0.1 atm). As shown in FIG. 7, when the temperature was changed from 527° C. to 727° C., the treatment time was decreased from about 185 min to about 10 min. FIG. 8 illustrates the decrease in ZrO₂ particle size (initial size=1 μm) for different initial particle sizes at 727° C. for CCl₄ concentrations of 0.1 atm (i.e., 10%, per volume, in carrier gas). This figure indicates that the treatment time needs to be chosen appropriately, given the particle sizes present in the soot. For example, if one knows that silica soot contains ZrO₂ particles that are up to 2 μm in diameter if the treatment temperature is 727° C., the processing time with the CCl₄ (assuming a concentration of 10% (or 0.1 atm)) is less than 20 min. If the ZrO₂ particles have a maximum diameter of Sum, the processing time with the CCl₄ (assuming concentration of 0.1 atm) is about 45-50 min. If the ZrO₂ particles have a maximum diameter of 10 μm, the processing time with CCl₄ (assuming a concentration of 10%) is about 90-95 min. However, these processing times can be decreased if the concentration of CCl₄ is increased.

Experimental Data:

Two grams of silica soot was placed inside a tube furnace equipped with a fused silica muffle. A flow of 2.36% CCl₄ in He was established and the furnace was ramped 10° C./min to 950° C. An infrared spectrum of the gas products downstream of the silica soot sample was acquired every 60 seconds. The results show that:

Below 400° C.: no reaction, only desorption of water from soot.

400° C.-600° C.: drying/doping of soot:

H₂O+CCl₄→2HCl+COCl₂ (drying)

2H₂O+CCl₄→4HCl+CO₂ (drying)

SiO₂+CCl₄→COCl₂+SiOxCly (doping)

600° C.-850° C.: radical decomposition of CCl₄:

CCl₄→:CCl₂+2.Cl→C₂Cl₄+Cl₂ (decomposition and recombination)

Above 850° C.: etching of SiO₂, carbonization:

SiO₂+CCl₄→SiCl₄+CO₂ (etching)

CCl4→C+4.Cl (carbonization)

This data indicates that in order to (i) maximize etching and volatilization of Cr₂O₃ and ZrO₂ and (ii) minimize etching of silica (muffle and preform) and carbonization, it is preferable to carry out the CCl₄ treatment of soot at about 800° C. Carbonization would not be a serious issue if the carbon stayed within the muffle, but the experimental data indicate that the carbon particles formed are small enough to be entrained outside of the muffle and deposit inside the piping downstream of the furnace, which could cause maintenance issues later on.

In one experiment, 8.7 g of silica soot doped with 1 wt % Cr₂O₃ (average particle size of 2 μm), 1 wt % of Fe₂O₃ (average particle size of 3 μm), and 1 wt % of ZrO₂ (average particle size of 5 μm) was pressed into a silica soot pellet with a density of 0.90 g/cm³ and treated with a mixture of 5% CCl₄ in He (this embodiment utilized He as carrier gas) at 800° C. for 45 minutes. The chemical analysis of the resulting pellet showed 0.53 wt % of Cr₂O₃, 0.13 wt % of Fe₂O₃, and 0.062 wt % or ZrO₂. This experimental data further demonstrates the effectiveness of CCl₄ at removing ZrO₂, and in reducing the amount and size of Fe₂O₃ and Cr₂O₃ particles in silica soot.

We discovered that for ZrO₂ particles of size d_(p) (cross sectional length, or diameter, in μm) treated at temperature T (in ° K) with mixture of carbon tetrachloride gas having concentration of y_(CCl4) (in atm), the treatment time should be greater than the diffusion time and reaction time, as shown below, i.e.:

t _(treatment,ZrO2) (in min)>t _(diffusion) +t _(reaction,ZrO2)  [4]

where the diffusion reaction time is a function of soot layer thickness L (in cm) and diffusion rate of the CCl₄ D_(eff,CCl4) (in cm²/sec) through the porous soot preform and is given as:

$\begin{matrix} {{t_{diffusion}\left( {{in}\mspace{14mu} \min} \right)} = \frac{L^{2}}{60D_{{eff},{{CCl}\; 4}}}} & \lbrack 5\rbrack \end{matrix}$

and the reaction time is given as:

$\begin{matrix} {{t_{{reaction},{{ZrO}\; 2}}\left( {{in}\mspace{14mu} \min} \right)} = {\frac{5.75 \times 10^{- 6}{d_{p}\left( {{in}\mspace{14mu} {\mu m}} \right)}{{Exp}\left\lbrack {12000/T} \right\rbrack}}{y_{{CCl}\; 4}\left( {{in}\mspace{14mu} {atm}} \right)}.}} & \lbrack 6\rbrack \end{matrix}$

According to some embodiments the treatment time with the CCl₄ is greater than 2 min, for example 5 minutes to 100 hours. Preferably the treatment time with the CO/Cl₂ mixture is 10 min to 15, for example 20 min, 30 min, 50 min, 1 hr, 1.5 hrs, 2 hrs, 3 hrs, 4 hrs, 6 hrs, 8 hrs, 10 hrs, 12 hrs, or some period of time there between.

The invention is further illustrated by the following examples depicted in Tables 1 and 2. Examples 1-54 of Table 1 show the total treatment times for the 6 cm thick silica based soot layers with maximum initial Cr₂O₃ particles of different sizes, when treated with different combinations of chlorine and carbon monoxide mixtures at various treatment temperatures, such that the size of the Cr₂O₃ particles was reduced to zero (i.e., no Cr₂O₃). As illustrated in these examples, the diffusion times are much smaller than the reaction times, and the total treatment time is almost equal to the reaction time. Examples 55-90 of Table 2 show the total treatment times (resulted in reduction of ZrO₂ particles sizes to zero) for the 6 cm thick soot layers with initial ZrO₂ particles of different sizes, when treated with different concentrations of carbon tetrachloride at various treatment temperatures. Many soot bodies articles include both of these impurities and therefore would need treatment with CCl₄ (between 600° C. and 850° C.) and CO+Cl₂ mixture (between 900° C. and 1200° C.). It is also noted that the in some applications total elimination of Cr₂O₃ and ZrO₂ particles may not be necessary, as long as their diameter is decreased below a critical size. For example, in fiber preforms, one may decide to simply decrease the particle sizes to a maximum size of about 0.2 μm or less, or to 0.1 μm or less, so that they are unlikely to become a cause of fiber breaks.

TABLE 1 Treatment of Cr₂O₃ containing soot with CO/Cl₂mixture Soot Soot Initial Total Treatment Layer Layer Cr₂O₃ Chlorine Carbon Diffusion Reaction Treatment Example Temperature Thickness Density Diffusivity, Particle concent. monoxide Time Time Time # [C.] (cm) (g/cm³) cm²/sec Size (μm) (atm) (atm) xCl₂ (min) (min) (min) 1 900 6.00 0.60 1.01 0.25 0.1 0 1.00 0.59 1354.87 13 55 2 900 6.00 0.60 1.01 0.25 0.1 0.05 0.67 0.59 168.51 169 3 900 6.00 0.60 1.01 0.25 0.1 0.1 0.50 0.59 142.62 143 4 900 6.00 0.60 1.01 0.25 0.1 0.2 0.33 0.59 151.22 152 5 900 6.00 0.80 0.64 0.25 0.1 0 1 0.94 1354.87 1356 6 900 6.00 0.60 1.01 0.25 0.2 0 1 0.59 677.43 678 7 900 6.00 0.60 1.01 0.25 0.2 0.1 0.67 0.59 84.25 85 8 900 6.00 0.60 1.01 0.25 0.2 0.2 0.5 0.59 71.31 72 9 900 6.00 0.60 1.01 0.25 0.2 0.4 0.33 0.59 75.61 76 10 900 6.00 0.60 1.01 1 0.1 0.1 0.50 0.59 570.47 571 11 900 6.00 0.60 1.01 1 0.2 0.2 0.5 0.59 285.23 286 12 900 6.00 0.60 1.01 1 0.4 0.4 0.5 0.59 142.62 143 13 900 6.00 0.60 1.01 5 0.1 0.1 0.5 0.59 2852.35 2853 14 900 6.00 0.60 1.01 5 0.2 0.2 0.5 0.59 1426.17 1427 15 900 6.00 0.60 1.01 5 0.4 0.4 0.5 0.59 713.09 714 16 900 6.00 0.60 1.01 10 0.1 0.1 0.5 0.59 5704.70 5705 17 900 6.00 0.60 1.01 10 0.2 0.2 0.5 0.59 2852.35 2853 18 900 6.00 0.60 1.01 10 0.4 0.4 0.5 0.59 1426.17 1427 19 1000 6.00 0.60 1.09 0.25 0.1 0 1.00 0.55 606.58 607 20 1000 6.00 0.60 1.09 0.25 0.1 0.05 0.67 0.55 75.44 76 21 1000 6.00 0.60 1.09 0.25 0.1 0.1 0.50 0.55 63.85 64 22 1000 6.00 0.60 1.09 0.25 0.1 0.2 0.33 0.55 67.70 68 23 1000 6.00 0.80 0.68 0.25 0.1 0 1 0.88 606.58 607 24 1000 6.00 0.60 1.09 0.25 0.2 0 1 0.55 303.29 304 25 1000 6.00 0.60 1.09 0.25 0.2 0.1 0.67 0.55 37.72 38 26 1000 6.00 0.60 1.09 0.25 0.2 0.2 0.5 0.55 31.93 32 27 1000 6.00 0.60 1.09 0.25 0.2 0.4 0.33 0.55 33.85 34 28 1000 6.00 0.60 1.09 1 0.1 0.1 0.50 0.55 255.40 256 29 1000 6.00 0.60 1.09 1 0.2 0.2 0.5 0.55 127.70 128 30 1000 6.00 0.60 1.09 1 0.4 0.4 0.5 0.55 63.85 64 31 1000 6.00 0.60 1.09 5 0.1 0.1 0.5 0.55 1277.00 1278 32 1000 6.00 0.60 1.09 5 0.2 0.2 0.5 0.55 638.50 639 33 1000 6.00 0.60 1.09 5 0.4 0.4 0.5 0.55 319.25 320 34 1000 6.00 0.60 1.09 10 0.1 0.1 0.5 0.55 2554.00 2555 35 1000 6.00 0.60 1.09 10 0.2 0.2 0.5 0.55 1277.00 1278 36 1000 6.00 0.60 1.09 10 0.4 0.4 0.5 0.55 638.50 639 37 1150 6.00 0.60 1.19 0.25 0.1 0 1.00 0.50 224.57 225 38 1150 6.00 0.60 1.19 0.25 0.1 0.05 0.67 0.50 27.93 28 39 1150 6.00 0.60 1.19 0.25 0.1 0.1 0.50 0.50 23.64 24 40 1150 6.00 0.60 1.19 0.25 0.1 0.2 0.33 0.50 25.06 26 41 1150 6.00 0.80 0.75 0.25 0.1 0 1 0.80 224.57 225 42 1150 6.00 0.60 1.19 0.25 0.2 0 1 0.50 112.28 113 43 1150 6.00 0.60 1.19 0.25 0.2 0.1 0.67 0.50 13.96 14 44 1150 6.00 0.60 1.19 0.25 0.2 0.2 0.5 0.50 11.82 12 45 1150 6.00 0.60 1.19 0.25 0.2 0.4 0.33 0.50 12.53 13 46 1150 6.00 0.60 1.19 1 0.1 0.1 0.50 0.50 94.55 95 47 1150 6.00 0.60 1.19 1 0.2 0.2 0.5 0.50 47.28 48 48 1150 6.00 0.60 1.19 1 0.4 0.4 0.5 0.50 23.64 24 49 1150 6.00 0.60 1.19 5 0.1 0.1 0.5 0.50 472.77 473 50 1150 6.00 0.60 1.19 5 0.2 0.2 0.5 0.50 236.38 237 51 1150 6.00 0.60 1.19 5 0.4 0.4 0.5 0.50 118.19 119 52 1150 6.00 0.60 1.19 10 0.1 0.1 0.5 0.50 945.54 946 53 1150 6.00 0.60 1.19 10 0.2 0.2 0.5 0.50 472.77 473 54 1150 6.00 0.60 1.19 10 0.4 0.4 0.5 0.50 236.38 237

TABLE 2 Treatment of ZrO₂ containing soot with CCl₄/Reaction of CCl₄ with ZrO₂ Soot Soot Initial Total Treatment Layer Layer ZrO₂ CCl₄ Diffusion Reaction Treatment Example Temperature Thickness Density Diffus^(ivity,) Particle conc. Time Time Time # [C.] (cm) (g/cm³) ^(cm2)/sec Size (um) (atm) (min) (min) (min) 55 600 6.00 0.60 0.61 1 0.1 0.98 53.72 54.70 56 600 6.00 0.80 0.42 1 0.1 1.43 53.72 55.14 57 600 6.00 0.60 0.61 1 0.2 0.98 26.86 27.84 58 600 6.00 0.60 0.61 2 0.1 0.98 107.43 108.42 59 600 6.00 0.60 0.61 2 0.2 0.98 53.72 54.70 60 600 6.00 0.60 0.61 2 0.4 0.98 26.86 27.84 61 600 6.00 0.60 0.61 5 0.1 0.98 268.58 269.57 62 600 6.00 0.60 0.61 5 0.2 0.98 134.29 135.27 63 600 6.00 0.60 0.61 5 0.4 0.98 67.15 68.13 64 600 6.00 0.60 0.61 10 0.1 0.98 537.16 538.15 65 600 6.00 0.60 0.61 10 0.2 0.98 268.58 269.57 66 600 6.00 0.60 0.61 10 0.4 0.98 134.29 135.27 67 700 6.00 0.60 0.69 1 0.1 0.87 13.08 13.95 68 700 6.00 0.80 0.469 1 0.1 1.28 13.08 14.36 69 700 6.00 0.60 0.69 1 0.2 0.87 6.54 7.41 70 700 6.00 0.60 0.69 2 0.1 0.87 26.16 27.03 71 700 6.00 0.60 0.69 2 0.2 0.87 13.08 13.95 72 700 6.00 0.60 0.69 2 0.4 0.87 6.54 7.41 73 700 6.00 0.60 0.69 5 0.1 0.87 65.39 66.26 74 700 6.00 0.60 0.69 5 0.2 0.87 32.70 33.57 75 700 6.00 0.60 0.69 5 0.4 0.87 16.35 17.22 76 700 6.00 0.60 0.69 10 0.1 0.87 130.79 131.66 77 700 6.00 0.60 0.69 10 0.2 0.87 65.39 66.26 78 700 6.00 0.60 0.69 10 0.4 0.87 32.70 33.57 79 850 6.00 0.60 0.805 1 0.1 0.75 2.52 3.26 80 850 6.00 0.80 0.53 1 0.1 1.13 2.52 3.65 81 850 6.00 0.60 0.805 1 0.2 0.75 1.26 2.00 82 850 6.00 0.60 0.805 2 0.1 0.75 5.04 5.78 83 850 6.00 0.60 0.805 2 0.2 0.75 2.52 3.26 84 850 6.00 0.60 0.805 2 0.4 0.75 1.26 2.00 85 850 6.00 0.60 0.805 5 0.1 0.75 12.59 13.34 86 850 6.00 0.60 0.805 5 0.2 0.75 6.30 7.04 87 850 6.00 0.60 0.805 5 0.4 0.75 3.15 3.89 88 850 6.00 0.60 0.805 10 0.1 0.75 25.19 25.93 89 850 6.00 0.60 0.805 10 0.2 0.75 12.59 13.34 90 850 6.00 0.60 0.805 10 0.4 0.75 6.30 7.04

Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that any particular order be inferred.

It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the invention. Since modifications combinations, sub-combinations and variations of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and their equivalents. 

1. A method of cleaning silica-based soot or an article made of silica-based soot, said method comprising: treating said silica-based soot or said article made of silica-based soot with at least one of the following compounds: (i) a mixture of CO and Cl₂ in a carrier gas such that the total concentration of CO and Cl₂ in said mixture is greater than 10%, by volume and the ratio of CO:Cl₂ is between 0.25 and 5; (ii) CCl₄, in a carrier gas, such that the concentration of CCl₄ is greater than 1%, by volume.
 2. The method of cleaning silica-based soot or an article made of silica-based soot according to claim 1, wherein the ratio of CO:Cl₂ is between 0.5 and
 2. 3. The method of cleaning silica-based soot or an article made of silica-based soot according to claim 1, wherein the ratio of CO:Cl₂ is between 0.75 and 1.5.
 4. The method of cleaning silica-based soot or an article made of silica-based soot according to claim 1, wherein the treatment with a mixture of CO and Cl₂ is performed between 900° C. and 1200° C.
 5. The method of cleaning silica-based soot or an article made of silica-based soot according to claim 4, wherein the treatment with a mixture of CO and Cl₂ is performed between 1000° C. and 1200° C.
 6. The method of cleaning silica-based soot or an article made of silica-based soot according to claim 4, wherein the treatment with a mixture of CO and Cl₂ is performed between 1100° C. and 1200° C.
 7. The method of cleaning silica-based soot or an article made of silica-based soot according to claim 1, wherein the treatment with CCl₄ is performed between 600° C. and 850° C.
 8. The method of cleaning silica-based soot or an article made of silica-based soot according to claim 7, wherein the treatment with CCl₄ is performed between 750° C. and 850° C.
 9. The method of cleaning silica-based soot or an article made of silica-based soot according to claim 2, wherein the treatment with a mixture of CO and Cl₂ is performed for at least 120 min.
 10. The method of cleaning silica-based soot or an article made of silica-based soot according to claim 9, wherein the treatment with a mixture of CO and Cl₂ is performed for at least 100 min.
 11. The method of cleaning silica-based soot or an article made of silica-based soot according to claim 9, wherein the treatment with a mixture of CO and Cl₂ is performed for at least 200 min.
 12. The method of cleaning silica-based soot or an article made of silica-based soot according to claim 11, wherein the treatment with a mixture of CO and Cl₂ is performed for at least 600 min.
 13. The method of cleaning silica-based soot or an article made of silica-based soot according to claim 2, wherein the treatment time with the CO/Cl₂ mixture is at least: t _(treatment,Cr2O3) (in mm)>t _(diffusion) +t _(reaction,Cr2O3) where the diffusion reaction time is a function of soot layer thickness L (in cm) and diffusion rate of the CO/Cl₂ mixture D_(eff) (in cm²/sec) through the porous soot preform, or loose silica soot and ${{t_{diffusion}\left( {{in}\mspace{14mu} \min} \right)} = \frac{L^{2}}{60D_{eff}}};$ and the reaction time is ${t_{{reaction},{{Cr}\; 2O\; 3}}\left( {{in}\mspace{14mu} \min} \right)} = \frac{4.3 \times 10^{- 4}\left( {d_{p}\left( {{in}\mspace{14mu} {\mu m}} \right)} \right){{Exp}\left\lbrack {12000/{T\left( {{in}\mspace{14mu} K} \right)}} \right\rbrack}}{y_{{Cl}\; 2}{x_{{Cl}\; 2}\left( {1 - {1.35x_{{Cl}\; 2}} + {0.372x_{{Cl}\; 2}^{2}}} \right)}}$ where x_(Cl2) is (y_(Cl2))/(y_(Cl2)+y_(CO)), and y_(cl2) and y_(co) are the partial pressure of chlorine and carbon monoxide respectively
 14. The method of cleaning silica-based soot or an article made of silica-based soot according to claim 7, wherein the treatment time with CCl₄ is at least: t _(treatment,ZrO2) (in min)>t _(diffusion) +t _(reaction,ZrO2) where the diffusion reaction time is a function of soot layer thickness L (in cm) and diffusion rate of the CCl₄ D_(eff,CCl4) (in cm²/sec) through the porous soot preform and is ${t_{diffusion}\left( {{in}\mspace{14mu} \min} \right)} = \frac{L^{2}}{60D_{{eff},{{CCl}\; 4}}}$ and the reaction time is given as: ${t_{{reaction},{{ZrO}\; 2}}\left( {{in}\mspace{14mu} \min} \right)} = {\frac{5.75 \times 10^{- 6}{d_{p}\left( {{in}\mspace{14mu} {\mu m}} \right)}{{Exp}\left\lbrack {12000/T} \right\rbrack}}{y_{{CCl}\; 4}\left( {{in}\mspace{14mu} {atm}} \right)}.}$
 15. The method of cleaning silica-based soot or an article made of silica-based soot according to claim 7, wherein the treatment time with CCl₄ is at least 20 min.
 16. The method of cleaning silica-based soot or an article made of silica-based soot according to claim 15, wherein the treatment time with CCl₄ is at least 50 min.
 17. The method of cleaning silica-based soot or an article made of silica-based soot according to claim 15, wherein the treatment with CCl₄ is at least 90 min.
 18. A method of cleaning silica-based soot or an article made of silica-based soot, said method comprising the following steps: treating said silica-based soot or said article made of silica-based soot with (i) CCl₄ in a carrier gas, such that the concentration of CCl₄ is greater than 1%, by volume; and (ii) a mixture of CO and Cl₂ such that the total concentration of CO and Cl₂ in said mixture is greater than 10%, by volume and the ratio of CO:Cl₂ is between 0.25 and 5; wherein said treatment with CCl₄ is performed either before, or after the treatment with the mixture of CO and Cl₂.
 19. The method of cleaning silica-based soot or the article made of silica-based soot according to claim 18, wherein said treatment with CCl₄ is performed at temperatures between 600° C. and 850° C., and said treatment with a mixture of CO and Cl₂ is performed at temperatures between 900° C. and 1200° C.
 20. The method of cleaning silica-based soot or the article made of silica-based soot according to claim 18, wherein said treatment with CCl₄ is performed for at least 2 min, and said treatment with a mixture of CO and Cl₂ is performed for at least 5 min. 